htaccess rewrite config for wordpress on its own folder and other folder apps.

I have read a lot of info about the htacess (here) but nothing related to the installation of wordpress on its own folder.
Also i tried those suggestion i posted and they are not working for me.

I have this setup:
/ – this is empty, just the index.php file from the wordpress installation and the htaccess file
/app/ this is an external app. The folder i want to access.
/app2/ another folder for another app
/wordpress/ this is my wordpress instalation.

I want to access the app folder and keep accessing the blog from the root.
This is my htaccess file on the root folder (and it is the same on the wordpress folder):